Ligaments and tendons are bands or sheets of fibrous connective tissue which provide support and stability to the musculoskeletal system. Unfortunately, injury and damage to a ligament or tendon is a frequent occurrence. Such injuries include those to the shoulder rotator cuff and ligaments of the knee, such as the anterior cruciate, and medial and lateral collateral, and acromioclavicular separations. Additional injuries include those to the lateral collateral ligaments of the ankle and to the large tendons, such as the achilles, quadriceps and patellar tendons, and the small tendons, such as flexor and extensor tendons of the hand.
Relief of the pain and/or instability caused by damage to a ligament or tendon is currently achieved by techniques ranging from simple suturing to removal and replacement with other tissue or a permanent synthetic prosthesis, depending upon the severity of the damage.
The state-of-the-art in severe ligament repair/reconstruction is considered to be the use of autogenous and allogeneic tissue grafts for augmentation or replacement of the damaged ligament. As an alternative to autogenous and allogeneic tissue grafts, xenografts, tissue grafts from a species other than the recipient species, have been implanted to replace natural ligaments. Xenografts have tended to be unpredictable in the long term for restoring full strength and stability to the involved joint.
Portions of the patellar tendon, iliotibial band, semitendinosus tendon, and fascia lata are some of the most commonly used autogenous and allogeneic tissue grafts. An example of such a procedure in the knee is the reconstruction of the anterior cruciate ligament by using a portion of the patellar tendon. Other tendons such as the semitendinosus tendon, and connective tissues such as fascia lata, are sometimes used to reconstruct the damaged ligament. Due to the undesirability of having to sacrifice one tissue and its associated function, in order to repair another, a number of synthetic, permanent total ligament prostheses and ligament augmentation implants have been tried. The medical community has prescribed certain characteristics for prosthetic ligaments. While all the properties influencing the ultimate success of a ligament prosthesis have not yet been defined, the following are some of the most important desired characteristics:
1. Adequate strength;
2. Resistance to elongation;
3. Fixation methods, the device should be easy to implant and attach;
4. Biocompatability, as demonstrated by a minimum of inflammatory responses;
5. Longevity, the device should last the lifetime of the patient;
6. Tissue ingrowth, host tissue should be able to penetrate the device to stabilize and ultimately enhance the device's physical property;
7. Activity, the implanted device should allow early if not immediate use of the limb;
8. Pliability;
9. Resistance to abrasion.
Several permanent, nonaugmented prosthetic ligaments have been developed. A permanent prosthesis is one which assumes its full strength initially upon implantation, is not intended to gradually resorb or disintegrate over time and does not depend on autographs or "regrown" natural ligament tissue for its ultimate success. Dore et al. U.S. Pat. No. 4,301,551, which issued on Nov. 24, 1981 describes a deformable silicone core surrounded by a tensionable wrapping of polymeric or stainless steel threads wound in a helical angle about the core. The core is the load bearing member and is capable of large elastic deformation in response to compression by the threads when the device is stretched. Two rigid plastic or stainless steel rods, one at each end of the core, connect the device to the bones of a joint.
A number of techniques employing carbon fiber-type or polypropylene augmentation devices are described in Vol. 196 of CLINICAL ORTHOPAEDICS AND RELATED RESEARCH, SYNTHETIC LIGAMENTS AND TENDONS, (H. Alexander & A. Weiss eds. June 1985). For example, a flat strap-like braid of polypropylene fibers was used to augment natural tissue grafts in studies conducted on goats. See G. McPherson et al. "Experimental Mechanical and Histologic Evaluation of the Kennedy Ligament Augmentation Device", CLINICAL ORTHOPAEDICS supra, p. 186. The time required for the recipient to return to normal activity is generally about one year or longer.
Treace U.S. Pat. No. 3,953,896, which issued on May 4, 1976 describes a prosthetic ligament made of a flexible, ultra high molecular weight polyethylene rod. Stainless steel sleeves and polyethylene nuts on each end of the flexible rod hold the prosthetic ligament to the bones.
Several of the permanent ligament prostheses are fabricated so that the properties of a single synthetic material characterize the implant's response to in vivo loading (see, e.g., U.S. Pat. Nos. 3,896,500; 3,953,896; 3,987,497; 3,988,783; and European Patent Application Nos. 51,954; 106,501; and 126,520, all of which are incorporated herein by reference).
Dahlen et al. U.S. Pat. No. 4,187,558, which issued on Feb. 12, 1980, describes a flexible braid made of Dacron (polyethylene terephthalate), encased in silicone rubber. A velour covered collar at one or both ends of the braid aids in attachment to the bone and promotes bone ingrowth to anchor the device.
A number of multi-component ligament prostheses (see, e.g. U.S. Pat. Nos. 3,797,047; 4,187,558; 4,483,023; and European Patent Application No. 122,744, all of which are incorporated herein by reference), are more bio-mechanically compatible with the elasticity and strength requirements of natural ligament function but suffer from other shortcomings. Since they are designed to replace the natural ligament, any reparative tissue that forms at the site of the defect, is almost completely shielded from applied loads and therefore tends to resorb.
Attempts at a long-term `natural` tissue repair (by augmenting but not replacing the natural tissue) has been approached by the use of a variety of devices and techniques. The use of a permanent device for augmentation of an autogenous tissue transplant is described in "Experimental Mechanical and Histologic Evaluation of the Kennedy Ligament Augmentation Device", G. K. McPherson, Ph.D. et al., Clinical Orthopedics and Related Research, Vol. 196, pages 186 to 195, 1985, which is incorporated herein by reference. While the method of attachment allows the desired natural tissue repair to occur, the entire synthetic implant remains in situ; some interfibrillar mechanical breakdown has been reported, and a chronic foreign body response is observed even at 2 years following implantation. A biologically mechanically degradable augmentation device consisting of polyglycolic acid (herein abbreviated as PGA) -coated carbon fibers (U.S. Pat. No. 4,411,027) or polylactic acid (herein abbreviated as PLA) - coated carbon fibers (U.S. Pat. No. 4,329,743) has also met with limited success in obtaining a `natural` tissue ligament repair.
Artificial ligaments and tendons made of bundles, interwoven or plated, and consisting of plastic fibers or of carbon fibers, coated with a substance absorbable inside the body are known in the art for tissue augmentation. For example, a flat strap-like braid of polypropylene fibers was used to augment natural tissue grafts in studies conducted on goats. See G. McPherson et al., "Experimental Mechanical and Histologic Evaluation of the Kennedy Ligament Augmentation Device", CLINICAL ORTHOPAEDICS, supra, p. 186. The time required for the recipient to return to normal activity is generally about one year or longer.
Another nonaugmented prosthetic ligament reported by C. Bolton and W. Bruchman, "The GORE-TEX Expanded Polytetrafluoroethylene Prosthetic Ligament", CLINICAL ORTHOPAEDICS, Vol. 196, p. 202, is constructed of bundles of Gore-Tex fibers arranged in a braided configuration. The braid is fixed by bone screws placed through eyelets at each end of the braid. Zachariades U.S. Pat. No. 4,587,163 discloses an isotropic semicrystalline morphology of ultra high molecular weight polyethylene for use in making artificial tendons or ligament protheses. The artificial tendon can be sutured to a natural tendon segment. The polyethylene described by Zachariades is an ultradrawn melt crystallized ribbon-like structure.
One device, described in U.S. Pat. No. 4,483,023 by Hoffman, et al., comprises a knitted polyester sheath surrounding a core of polyester strands.
U.S. Pat. No. 4,149,277 introduces another variant in the construction of prosthetic ligaments and tendons in the form of carbon coated polyester filaments in braided, woven or meshed array.
U.S. Pat. No. 4,585,458 describes the use of chemically fixed heterologous collagenous tissues instead of synthetic materials for repair or replacement of ligaments or tendons. At present such devices have not proven to be effective alternatives for ligament repair since they typically exhibit premature failure.
All of the aforementioned tissue replacement and/or augmentation approaches suffer from various deficiencies. A problem associated with the autogenous transplant methods for ligament reconstruction relates to damage and loss of strength of the donor structure. During the healing process these donor tissues eventually regain their strength. However, until the strength of the tissues is recovered, they must be protected from carrying normal loads. Therefore, these procedures are accompanied by long rehabilitation regimens.
The synthetic prosthetics also suffer from certain deficiencies. While ligamentous tissue is a natural composite material exhibiting both compliant elasticity and high longitudinal strength, no single synthetic biocompatible material has this combination of properties. The inevitable chemical and/or physical breakdown of these implants in vivo, leads to catastrophic failure and a return to pre-operative instability, or worse, because no natural tissue repair has taken place. As a result, implants such as the ones listed above have tended to fail in animal or clinical trials either by material fatigue, creep, or in-vivo degradation leading to joint laxity, or by unacceptable restriction of joint motion.
As mentioned above, one problem often experienced by recipients of some prosthetic ligaments is that the braided ligaments undergo constructional deformation after implantation and, as a result, become too lax over time. Constructional deformation occurs when the fibers of the braided ligament prosthesis compact and undergo helical changes. There is a certain amount of "slack" in the prosthetic ligaments heretofore available which permits them to undergo constructional deformation with use. The prosthetic ligament lengthens and loses its tensioning capacity.
Also, as a result of the contact of yarns in braided, woven, or meshed constructions, ultimately a delamination of carbon from these points could occur, thus reducing the biocompatability of the device.
The mechanical properties of carbon fibers used at present predominantly for implants are strongly non-isotropic, due to the graphitic structure of carbon. Bending loads or stresses with small radii of curvature, or even relatively slight shearing stresses, may cause fibers to fracture. Furthermore, carbon fibers have only low ductility and tend to crumble, particularly if their coats have been dissolved after a longer stay inside the body.
No ligament prosthesis, tried thus far in animals or humans, has yielded consistently acceptable joint stability without the occurrence of implant breakdown, synovitis, and/or articular tissue damage during the first two years post operatively. The desired minimum post operative period of implant/joint stability is 10 years.
It is clear that there remains a significant need for a method for achieving a ligament replacement or augmentation having initial and long-term strength, flexibility, extension and recovery that at least approximate those of the original, undamaged ligament or other fibrous connective tissue.